For the automobile brake system, with an increased development of a control system mounted on the automobile and a requirement of stability during driving of an Anti-skid Brake System, referred to as “ABS” hereinafter, and a traction control, instead of a braking operation of a driving person, improved systems for the braking operation of the wheels are available so that an electronic control system reads and then determines an optimal braking condition from the operation of the driving person. Also, some of the systems are equipped with an auxiliary pressure source for the generation of a higher braking pressure in addition to a mechanism for the generation of the braking pressure due to a biasing force applied to a brake pedal by the driving person.
JP 10-86802 (A) discloses an example of the above-described brake system, in which a signal that indicates a braking pressure generated by the braking operation of the driving person is transmitted to the controller. In response to the signal, the controller controls a separate pressure generator so that the generator generates and then supplies a braking liquid-pressure to the wheel systems. According to the disclosed system, the liquid pressure generated by the braking operation of the driving person is used only for the generation of the control signal but not used directly for the braking control of the wheel systems.
Also, JP 10-35455 (A) discloses another control system. According to the control system, the braking force is calculated based upon a stroke of the pedal. A result of the calculation is then used for an adjustment of a high pressure, generated by the auxiliary pressure source, at a proportional pressure control valve so that a suitably adjusted pressure is transmitted to the wheel systems. In this system, the liquid pressure generated by the pedal operation by the driving person is also used for the braking operation of the wheels.
With reference to the drawings, a conventional brake system for automobiles will be described below. FIG. 11 illustrates an example of the automobile brake system with an auxiliary pressure source. In this drawing, reference numeral 1 indicates a brake pedal on which the driving person applies a biasing force for the brake control of the automobile. Also, the reference numeral 2 indicates a device (hereinafter referred to a “master cylinder”) for generating a pressure force in the braking substrate or liquid according to the braking operation of the driving person. For this purpose, the master cylinder 2 is fluidly connected with two passages or tubes 3 and 4 to a reservoir 5 for the accommodation of a braking substrate or liquid.
The master cylinder 2, which is a tandem cylinder with two pistons arranged in a serial fashion within a cylinder, is capable of providing the pressurized liquid in two ways or passages 6 and 16 fluidly connected at respective ends to respective pistons. The passage 6 is fluidly connected to a switching valve 7 and then branched into two sub-passages 8 and 9. Normally, an electromagnetic valve is preferably used for the valve 7, although another type of valve may be employed instead. The branched passage 8 is fluidly connected with a right-front brake unit 10 and the other branched passage 9 is fluidly connected with a left-rear brake unit 11, for example. When the electromagnetic valve 7 is electrically disconnected from a power source, it is fluidly connected with the passage at its portion, e.g., left portion indicated by an arrow with arrowheads at its opposite ends in the drawing, allowing the liquid in the passage 6 to pass therethrough. On the other hand, when the electromagnetic valve 7 is electrically connected with the power source, it is connected with the passage at its another portion, e.g., right portion, prohibiting the liquid in the passage 6 from passing therethrough. Also, sensors 13 and 14 for sensing a pressure of the liquid in the passage 6 are provided in the passage 6 and on the opposite sides of the valve 7.
Likewise, the passage 16 is connected to an electromagnetic valve 17 and then branched into sub-passages 18 and 19 which are in turn connected with left-front brake unit 20 and right-rear brake unit 21, respectively. The electromagnetic valve 17 has the same structure and connection as those of the valve 7. Also, sensors 23 and 24 are arranged on opposite sides of the valve 17 for the sensing of the pressure in the liquid passing therethrough. Further, although the automobile brake system includes other passages and valves for ABS and traction control, they are eliminated from the drawing for clarity.
In operation of the brake system so constructed, in the state shown in FIG. 11 when the valves are electrically disconnected from the power source, the displacement of the pedal 1 due to the biasing operation of the driving person pressurizes the braking liquid accommodated in the two pistons in the master cylinder 2, which increases the pressure in the passages. The pressure in the passage 6 is transmitted through the electromagnetic valve 7 and then the passages 8 and 9 into brake units in the right-front and left-rear wheel units 10 and 11, respectively, where it is used for the braking control of respective wheels. Likewise, the pressure in the passage 16 is transmitted through the electromagnetic valve 17 and passages 18 and 19 into brake units in the left-front and right-rear wheel units 20 and 21, respectively, where it is used for the braking control of respective wheels.
Afterwards, when the biasing force is removed from the brake pedal, the pressure is removed from the pistons in the master cylinder 2. This also removes the pressure in the passages 6 and 16, which releases the wheels from the braking control thereof.
Descriptions will be made to other parts including an auxiliary pressure source of the brake system. The auxiliary pressure source generally indicated by reference numeral 30 includes a motor 31 with an output shaft connected with two pumps 32. Each pump 32 is fluidly connected at an inlet or suction port thereof through a passage 34 to the reservoir 5 and at an outlet or discharge port through a passage 35 to an accumulator 36 and a sensor 37 for sensing the pressure in the passage. A relief valve 38 is provided for the protection of a disadvantageous rapid increase in pressure within the passages. Also, the passage 35 is fluidly connected to an input port 41 of a proportional pressure control valve 40 which will be described below. The valve 40 has output and return ports 42 and 44 connected with passages 43 and 45, respectively. The passage 45 is branched from the passage 34 and then terminates at the reservoir 5.
The passage 43 connected at its one end to the output port 42 of the valve 40 is branched into a passage 46 which is extended through an electromagnetic valve 47 to the passages 8 and 9, and another passage 48 which is extended through an electromagnetic valve 49 to the passage 18 and 19. In the drawing, the electromagnetic valve 47 is illustrated to be closed when it is disconnected from the power source. An electronic control device 70 is provided so that it is electrically connected at its terminal indicated by alphabet A to terminals of the motor 31 and the valve 40, also indicated by alphabet A. For clarity, the connections thereof are eliminated from the drawing. In addition, although not illustrated in the drawing, the control device 70 is electrically connected to all of the sensors 13, 14, 23, 24 and 37 and the electromagnetic valves 7, 17, 47 and 49.
The operation of the auxiliary pressure source 30 and portions related thereto so constructed will be described hereinbelow. Upon rotation of the motor 31, the pumps 32 are driven to draw the brake liquid from the reservoir 5 through the passage 34 into the accumulator 36. The accumulator 36 may be selected from various types of accumulators. For example, if the piston type accumulator which is made of cylindrical tube with two spring-biased pistons received therein is used, the brake liquid is forced in the accumulator 36 so that the springs are compressed to accumulate the braking force or energy therein. The adjacent pressure sensor 37 senses the braking pressure in the accumulator 36. Then, when the pressure has reached a predetermined value, the sensor 37 transmits a corresponding signal. The signal is then transmitted to the control device 70, which in turn de-energizes the motor 31.
The accumulated pressure decreases by the braking operations, which is sensed by the pressure sensor 37. When the pressure sensor 37 detects that the pressure has decreased to a predetermined value, it transmits a signal to the control device 70. Upon receiving the signal, the control device 70 drives the motor 31 to accumulate the pressure in the auxiliary pressure source 30. This means that the auxiliary pressure source 30 drives the pumps in response to the pressure decrease so that a certain amount of braking energy with the predetermined pressure is always accumulated therein whenever the automobile is electrically powered. The operation of the brake pedal 1 is not directly linked with the pump 32. This ensures that, even when the electric system in the automobile is disabled suddenly during driving, the auxiliary pressure source maintains therein a sufficient braking energy.
The pressure of the brake liquid is transmitted through the passage 35 to the input port 41 of the proportional pressure control valve 40. Then, by a control operation described below, the pressure is further transmitted through the output port 42 to the passage 43 and then branched passages 46 and 48 into the electromagnetic valves 47 and 49. While electrically disconnected from the power source as shown in FIG. 11, the electromagnetic valves 47 and 49 close and the electromagnetic valves 7 and 17 open, which allows the braking pressure to be transmitted through the master cylinder 2 to the control device. Once electrically connected with the power source, the electromagnetic valves 47 and 49 open and the electromagnetic valves 7 and 17 close. This allows the pressure of the braking liquid to be transmitted from the valve 47 through the passages 8 and 9 to the brake units of the right-front and left-rear wheels 10 and 11, and from the valve 49 through the passages 18 and 19 to the brake units of the left-front and right-rear wheels 20 and 21, respectively. This results in the four wheels being well braked. In this braking operation, the brake liquid is accumulated in the auxiliary pressure source 30 with an increased pressure that is higher than that transmitted directly from the master cylinder 2. This results in a greater braking force with less force applied on the pedal by the driver.
While connected with the power source, although not shown, the electromagnetic valves 7 and 17 provided in the respective passages 6 and 16 that connect the master cylinder 2 to the respective wheels are closed. This prevents the pressure generated in the master cylinder due to the biasing operation of brake pedal 1 from being transmitted to the wheels. When the system is electrically connected with the power source, a signal indicating the pressure in the master cylinder 2 is transmitted through the output terminals of the pressure sensors 13 and 23 to the control device 70. Once received, the control device 70 calculates a brake force required for the braking of the wheels in response to the input signal. Then, based upon the calculated brake force, the control device 70 transmits a signal to the valve 40 where the output pressure from the auxiliary pressure source 30 is adjusted to a certain level suitable for the braking control of respective wheels.
When an ignition of the automobile is turned off or the electric system thereof is in a malfunction condition due to any reason, all of the electromagnetic valves are turned off as illustrated in the drawing. In this instance, the breaking pressure caused by the braking operation of the pedal by the driving person is directly transmitted to the wheels 10, 11, 20 and 21. Therefore, although the high pressure in the auxiliary pressure source 30 is not used for the braking operation, a minimum braking force required during the emergency is ensured.
FIG. 12 illustrates an example of the proportional pressure control valve 40 and portions related thereto. It should be noted that like parts are denoted by like reference numerals throughout the drawings. Also, in this drawing, although the automobile has a plurality of wheels and brake systems therefor as shown in FIG. 11, only one wheel and its brake system is illustrated therein for clarity. Likewise, the sensors are also eliminated from the drawing.
In this drawing, the proportional pressure control valve 40 has an actuating or mechanical portion 50 and a control portion 60 indicated as upper and lower portions in the drawing, respectively. The mechanical portion 50 includes a sleeve 51 in the form of a cylinder which is closed at its uppermost end and opened at its lowermost end, and a spool 52 mounted in an interior of the sleeve 51 so that it can move in an axial direction of the sleeve 51 while making a sealing contact with an inner surface of the sleeve 51. The sleeve 51 has three ports; input port 41, output port 42 and return port 44, formed therein so that each of three ports fluidly communicates between interior and exterior thereof. As described above, the three ports are connected with first ends of the passages 35, 43 and 45, respectively.
The spool 52 is formed at its mid portion in the longitudinal direction with a reduced portion 53 which serves as a valve that connects and disconnects between the input port 41 and the return port 44 by a displacement of the spool 52 in the axial direction. The output port 42 communicates with a chamber defined adjacent to the reduced portion 53 irrespective of the position of the spool 52. The reduced portion 53 has a transverse hole 54 that extends in a direction perpendicular to and crossing the axial direction of the spool 52. The transverse hole 54 is fluidly connected with a vertical hole 55 that extends downwardly along the axial direction so that the holes 54 and 55 cooperate with each other to form a T-shaped passage in the spool 52. The vertical hole 55 receives a cylindrical pin 56 so that the pin moves in the axial direction with a sealing contact between an outer surface of the pin and an inner surface of the vertical hole.
The control portion 60, which is mounted in the lower part of the valve 40 and positioned in a coaxial fashion with the mechanical portion 50, has an outer cylindrical portion 61. The cylindrical portion 61 is closed at its lower end that defines one end of the valve 40 and is opened at its upper end that makes a sealing connection with the bottom end of the sleeve 51. An inner diameter of the cylindrical portion 61 is greater than that of the sleeve 51 to form a step at the connection thereof. A coil 62 is wounded around the cylindrical portion 61. Also, provided in the interior of the cylindrical portion 61 is a stop 63 in the form of rod that extends along the axial direction from the bottom end 64. Also, the top end of the stop 63 received the bottom end of the pin 56 to restrict a downward movement of the pin 56.
The spool 52 is formed at its bottom portion with an enlarged cylindrical portion 57 that extends downwardly in the interior of the cylindrical portion 61. The enlarged portion 57 cooperates with a portion of the spool 52 to define a step at an uppermost end of the enlarged portion. The step cooperates with another step formed between the sleeve 51 and the cylindrical portion 61 to define a stop therebetween that restricts an upward movement of the spool 52 and the enlarged portion 57. The enlarged portion 57 is formed from its bottom end with a hole that receives the rod-like stop 63. Also, the enlarged portion 57 is formed at its inner periphery surface with a step. A spring 65 is supported at its opposite ends by the step and the closed end of the cylindrical portion 61 so that it forces the spool 52 upward.
An operation of the proportional pressure valve 40 so constructed will be described hereinbelow. It should be noted that FIG. 12 is in part different from FIG. 11 and illustrates a powered condition, i.e., in which the ignition key is turned on. In this state, upon operation of the brake pedal 1 by the driver, the master cylinder generates a braking pressure that is transmitted to the passage 6. However, since the electromagnetic valve 7 is closed as shown in the powered condition, the pressure is never transmitted directly to the wheel 10. A signal indicative of the pressure in the master cylinder 2 is transmitted from the pressure sensor 13 to the control device 70. Based upon the signal, the control device 70 calculates a required braking pressure and then instructs the control 60 of the valve 40 to apply an electric current necessary for generating the required braking pressure. The coil 62 cooperates with the enlarged portion 57 of the spool 52 to form a solenoid, so that the enlarged portion 57 is attracted to a magnetic field generated by the application of the electric current to the coil 62. This causes the enlarged portion 57 together with the spool 52 to move downward against the biasing force by the spring 65.
With the downward movement of the spool 52, the return port 44 communicated with the reduced portion 53 is closed and then the input port 41 is brought into communication with the reduced portion 53. Since the input port 41 is connected through the passage 35 with the accumulator 36 for accumulating the brake liquid pressurized by the driving of the pump 32, the communication between the input port 41 and the reduced portion 53 allows the pressurized liquid to be fed from the passage 35 through the input port 41 and the peripheral chamber defined by the reduced portion 53 to the output port 42. Then, the liquid is fed through the electromagnetic valve 49 that is opened in the powered condition and through the passages 43 and 48 to the wheel 10.
When the brake pedal 1 is released by the driver, the pressure in the master cylinder 2 decreases, which is transmitted from the pressure sensor 13 to the control device 70. Then, the control device 70 turns off the application of the electric current to the coil 62. This de-energizes the solenoid to cause the spool 52 to move upward due to the force applied thereto by the spring 65. At this moment, the input port 41 is closed to prohibit the braking liquid from flowing into the reduced portion 53. Instead, the reduced portion 53 is connected to the return valve 44, which causes the braking pressure that has been applied to the wheel 10 to be released through the output port 42 into the return port 44.
Another description will be provided of a relationship between the displacement of the brake pedal and the braking pressure liquid pressure. As described above, the signal indicative of the pressure in the master cylinder 2, caused by the stroke of the brake pedal, is transmitted from the pressure sensor 13 to the control device 70. In response to the signal, the control device 70 instructs the valve control 60 of the proportional pressure control valve 40 to apply the electric current I that is proportional to the braking pressure. Typically, it has been known that the force for the solenoid to move the spool varies in proportion to the square of the current.
As described above, the spool 52 includes the transverse hole 54 and a vertical hole 55 in which the pin 56 is inserted. The lowermost end of the pin 56 is exposed to the interior of the cylindrical portion 61 where no braking pressure applies thereto. This means that an excessive pressure (P×S) acts in the transverse hole 54, wherein    P: Braking pressure introduced through input port 41;    S: Cross section of pin 56 (corresponding to cross section of vertical hole 55); and    F: Biasing force upwardly applied to the spool 52 by the spring 65.This results in that the spool 52 stays where an upward force that is the sum of the upward force (P×S) and the biasing force F of spring 65 balances to a downward force caused by the coil 61. This balanced state can be indicated by the following equation:P×S+F=a×I2 (a: Coefficient)This can be written as follows:P=[a×I2−F]/S 
This relation, which is graphed as shown in FIG. 13, means that the braking pressure is proportional to the square of the current or the biasing force applied to the brake pedal. This further means that the proportional pressure valve 40 uses the high braking pressure accumulated in the accumulator 36 in order to adjust the required braking pressure to be transmitted to the wheel, depending upon the stroke of the brake pedal.
FIG. 14 illustrates another conventional brake system. The system does not include means for transmitting the liquid pressure in the master cylinder 2, generated by the operation of brake pedal 1, to the wheel 10 directly. Alternatively, a signal indicative of the pressure in the liquid is transmitted to the control device 70. Then, a required pressure for braking is generated only by the operation of the valve 40 driven by the instruction from the control device 70.
As shown, the master cylinder 2 has a pressure sensor 13 for transmitting a signal indicative of the pressure to the control device 70. Also, the braking pressure to be used for braking the wheel 10 is obtained only from the proportional pressure control valve 40. This allows the electromagnetic valve for changing the passages to be eliminated, which simplifies the structure of the brake system. Other parts are the same as those described above and, therefore, no detailed description will be made thereto. With the arrangement, indeed the structure of the brake system can be simplified. However, since no means is included for applying a certain braking pressure to the wheels, a separate structure should be provided for safety during possible emergencies, such as any failure of the electric system.
As described above, for the conventional brake system, the pressure accumulated in the auxiliary pressure source is not used for the braking operation during emergencies caused by, for example, the electric or system failure in the automobile. That is, in the conventional brake system disclosed in JP 10-86802 (A), although the pressurized liquid is accumulated in the auxiliary pressure source, it is prohibited by the electromagnetic valve from being transmitted to the wheels during electric failure. Also, the pressure generated by the pedal operation is not transmitted to the wheels, which disadvantageously brings the automobile into a condition in which no braking pressure is applied to the wheels. To prevent this, another auxiliary brake system should be provided.
On the other hand, the brake system disclosed in JP 10-35455 (A) is designed so that the pressure in the liquid generated by the operation of the brake pedal is transmitted to the wheels even during emergencies in which no electric power is supplied to the brake system. This ensures a certain braking force, although it might be a minimum braking pressure caused by the driver, that must be ensured by the manufactures. During electric system failure, the pressure accumulated in the auxiliary pressure source is prevented by the electromagnetic valve from being transmitted to the wheels. This means that the pressure is not used effectively for the braking during emergencies.